A superconductive element or apparatus contained in a cryostat filled with a cryogenic liquid can transport high-current electricity without losses by the Joule effect. However, in order to supply high-current electricity to the superconductive element, it can be necessary to feed it under medium or high voltage. Below, when describing the present invention, the term “medium or high voltage” is used to designate voltages greater than about 50 kilovolts (kV).
Electricity is conveyed from a source situated in ambient air, and thus at ambient temperature, to the superconductive element situated in a cryostat, and thus at cryogenic temperature, by using a feed-through device known as a “bushing”, which mainly comprises a central electrical conductor surrounded by an insulating sheath. A bushing must accomplish the temperature transition over a reasonable length while ensuring that losses by thermal conduction are low so as to avoid too great a flow of heat being transmitted into the cryogenic liquid, thereby leading to it boiling. In addition, the bushing must be capable of conveying electricity at high current and it must be capable of withstanding high voltage.
In order to feed a superconductive element with electricity, it is necessary to use one bushing for current inflow and another bushing for current outflow. If the feed voltage is high, there is a risk of electrical discharge occurring between the two ends of the two bushings that are situated in the ambient air. In other words, the electric field between those two ends can become great enough to lead to an electric arc forming between the two ends that are situated in air, i.e. to a “breakdown” in air. That phenomenon occurs essentially when there is a fault in the superconductive element, or when its current-carrying capacity is exceeded, e.g. during a current surge on the network. Under such circumstances, electrical continuity between the two bushings is interrupted, at least temporarily, meaning that one of the bushings remains at the high voltage potential while the other is to be found at a potential close to ground potential. Spacing the two bushings apart by a distance that is sufficient to avoid a breakdown occurring, e.g. a distance of several meters, cannot be envisaged because of the dimensional constraints imposed on the cryostat containing the superconductive element. The size of the cryostat is limited for reasons of cost, both manufacturing cost and running cost. The present invention proposes a device that makes it possible to avoid an electric discharge appearing.
Another problem associated with feeding electricity that might be at high current and under high voltage, lies in the temperature of the cryogenic fluid being raised locally, and thus to gas forming, which might be due to a defect in the superconductive element, or more simply to the nature of said element. For example, if the superconductive element is a current limiter immersed in liquid nitrogen, the current limiter ceases to be superconductive if the magnitude of the current carried thereby exceeds a certain threshold. The current limiter then heats up by the Joule effect, thereby leading to local boiling of the liquid nitrogen, and thus to bubbles of nitrogen gas forming. Since nitrogen in gaseous form is much less electrically insulating than nitrogen in liquid form, an electric discharge can then occur within the cryostat, e.g. between the two ends of the two bushings that are situated inside the cryostat. An embodiment of the present invention provides a solution to this problem.